1. Field of the Invention
The invention relates to a two-thread lock-stitch sewing machine with a thread cutter, comprising the following features: a bearing plate for a workpiece; a stitch hole formed in the bearing plate or in the feeder for a sewing needle to pass through that carries a needle thread and reciprocates up and down along a motion travel; a bearing block is arranged underneath the bearing plate, having a shuttle rotatably lodged in it; the shuttle has a shuttle body that rotates about a vertical axis with a bobbin case rotatably accommodated therein inclusive of shuttle thread supplies; two stationary cams which are disposed in proximity to the stitch hole and a holding finger which is provided on the bobbin case and has an end located between the cams; the thread cutter with a thread pulling knife, which is movable to reciprocate between a swung-out position and a swung-in position, performing a cutting motion in an x-y plane that extends at right angles to the axis, which comprises a hook, which seizes the threads, and a cutting edge, and which, in the swung-in position, is movable into a space between the motion travel and the cams.
2. Background Art
A two-thread lock-stitch sewing machine of the generic type is known from U.S. Pat. No. 3,658,021. The thread cutter comprises a thread pulling knife pivotable in a plane that extends at right angles to the vertical axis of rotation of the shuttle. For a thread cutting process to take place, in which short tail pieces of the needle and shuttle threads remain on the lower side of the workpiece, a catch thread device and a cutting knife are moved in opposite directions, with the motion travel of the catch thread device being between the motion travel of the needle and the holding finger of the bobbin case.
The fact that the thread puller and the cutting knife are formed on the ends of sectors of an annulus demands for a complicated bearing arrangement. Additionally, this kind of a thread cutter needs a special drive mechanism. All this leads to an overall construction that is accompanied with a considerable manufacturing cost and consists of a considerable number of components, by which to achieve a mode of operation of only limited durability and reliability.
U.S. Pat. No. 3,211,117 teaches a thread cutter for sewing machines, in which a thread pulling knife is formed on the elbowed ends of a lever arm which is tightly united with a pivot axis that is parallel to the vertical axis of rotation of the shuttle. When a cutting motion is performed, the thread pulling knife is positioned in its swung-in position between the bobbin case and the holding finger that is disposed thereon. Due to this arrangement, the thread pulling knife finds itself at a comparatively great distance from the stitch hole so that tail pieces of a length of some millimeters are inevitable on the lower side of the workpiece after the needle and the shuttle threads have been cut.
It is an object of the invention to further develop the known two-thread lock-stitch sewing machine in such a way that a durably reliable mode of operation is attained, which is accompanied with a decrease in the manufacturing cost of the thread cutter.
This object is attained in the two-thread lock-stitch sewing machine by the following features: the thread pulling knife is formed on the free end of a lever arm which is connected to a shaft that is parallel to the axis, and which is movable above the bobbin case; the cams are disposed on a rib, which has an upper surface that is substantially parallel to the x-y plane; the bearing plate is provided with a lower surface that is substantially parallel to the x-y plane, with the lower surface and the upper surface being disposed at a distance from each other that is measured at right angles to the x-y plane, forming an interspace for the lever arm to pass through unimpeded for the performance of the cutting motion.
Providing an interspace between the cams that safeguard the bobbin case against rotation and the workpiece bearing plate enables a familiar thread pulling knife of simple structure to be used, which can be produced at a low cost and which, in the swung-in position with the to-be-cut threads seized, can be positioned in a space between the motion travel of the sewing needle and the cams. Simultaneously, the arrangement according to the invention helps achieve additional guidance of the thread pulling knife, which facilitates cutting jobs of thicker threads of a diameter exceeding 0.3 millimeters. The two-thread lock-stitch sewing machine and the thread cutter according to the invention are characterized by a durably reliable mode of operation.
The further development, according to which the rib is formed in the way of a bilaterally clamped support, permits rigid mounting of the rib that is provided with the cams. This has the further advantage that the workpiece bearing plate, with the cams missing on its lower side, may be produced from sheet material and thus at a lower cost. The effect of this cost advantage is even stronger when the bearing plate is provided with a stitch hole and thus subject to special wear.
The embodiment, according to which the plate and the bearing plate are provided with congruent drilled holes for the reception of fastening screws, enables the bearing plate and the plate that is provided with the cams to be fastened by the same fastening screws, precluding any additional measures for fixing the two plates. The further development, according to which the rib is formed on the workpiece bearing plate, results in a more rigid construction. Furthermore, integrating two components leads to reduced manufacturing costs and simplified handling during maintenance and repair. An advantageous further development is attained by the rib being formed in the way of a bilaterally clamped support.
Details of the invention will become apparent from the ensuing description of exemplary embodiments, taken in conjunction with the drawing.